The present invention relates to a method by which solid metal alloy castings such as ingots, billets and the like destined for thixotropic forming are brought to the semisolid or semiliquid state and to equipment for its implementation. In accordance with the invention the castings are typically alloys of aluminum, magnesium and copper.
A recent addition to the range of processes adopted, for the shaping of metal alloys, typically pressure diecasting, forming, and others, is the method known as thixotropic forming. Such a method employs ingots prepared from a metal alloy that is brought to the semisolid or semiliquid state before being shaped and the alloy exhibits a particular structure of a homogeneous arrangement of solid crystals, globules or granules immersed in a liquid phase. Accordingly, this type of process requires a partial or total fusion of those phases of the alloy with a lower melting point, while the globular phases determining the thixotropic nature of the alloy must be maintained in the solid state.
In practice, the resulting structure is composed of solid globules distributed homogeneously within a liquid phase, with no dendrites, i.e. devoid of crystals growing arborescently around nuclei. In this type of process it is essential that the proportions between solid phase and liquid phase are able to be reproduced at will in any ingot cast as a thixotropic starting material, compatibly with the type of alloy and the forming process adopted, so as to ensure that the behavior of the alloy in forming and the specifications of the end product can be maintained constant.
Referring to the accompanying drawings, the state of a metal alloy suitable for thixotropic forming is indicated schematically by the graph of FIG. 1, the part of the curve to the left of point A represents material entirely in the liquid state, whereas the part to the right of point C represents material entirely in the solid state. The parts of the curve between points A and C indicate the semisolid or semiliquid material and, more exactly, the part between B and C represents a material composed of solid crystals or granules or globules immersed in a liquid phase, which is the eutectic.
Progressing from point C to point B, the percentage of eutectic in the liquid state as opposed to solid crystals increases from 0 to 100. From point B to point A, on the other hand, it is the percentage of crystals in solid solution passing to the liquid state the increases from 0 to 100. In the case of thixotropic alloys, the areas of interest are generally B-C, where one has solid crystals together with eutectic in the liquid state, and apart of B-A depending on the liquid fraction effectively required.
An ingot of such a material will behave as a solid when conveyed or handled, but behaves in the manner of a liquid when subject to any type of forcible shaping operation.
In summary, an ingot in the thixotropic state is devoid of dendrites tending to jeopardize its homogeneous composition and mechanical strength.
Again referring to FIG. 1, and in particular to the part of the curve between B and C, it will be noted that the mere application of heat is not enough to induce the required semisolid or semiliquid state of the material. But, in practice, the material must be maintained at the requisite temperature for a given length of time.
Conventionally, an ingot of any given description in the solid state and at ambient temperature is brought to the semisolid or semiliquid state using induction furnaces, in which the heat is produced by generating a magnetic field whose flux lines directly envelop the ingot. The correct heating action, in terms of obtaining the requisite temperature and maintaining the ingots at the same temperature for the correct duration of time, usually is determined by trial and error, whereupon the conditions which are seen to produce the desired end result must be repeated exactly.
The typical induction furnace consists essentially of a cylindrical crucible accommodating a single ingot surrounded by induction coils disposed in such a manner as to generate a magnetic field with flux lines impinging on and enveloping the ingot.
Clearly, any variation in value and frequency of the magnetic field will cause a corresponding variation in the temperature applied to heat the ingot and cause a different distribution of heat between the skin and the core of the ingot. By regulating and monitoring the value of the magnetic field in the appropriate manner, the type of heating action applied to the ingot can be controlled selectively, targeting areas further and further in toward the ingot core.
The time taken by such furnaces to bring each ingot to the desired temperature will naturally depend on the dimensions of the ingot.
For a better illustration of the problem addressed by the present invention, reference may be made to a specific example: to bring an ingot some 150 mm in diameter and 380 mm in height to the semisolid or semiliquid state in the correct manner using an induction furnace of conventional type, a time of approximately 18 minutes is required. This may be acceptable in an experimental situation, but is not acceptable for industrial scale manufacture.
Considering a production rate of one ingot per minute as acceptable, a battery of 18 conventional furnaces would be required to achieve such a rate. First of all, there are serious problems of economy associated with the operation of so many furnaces, given their high overall power consumption. What is more, there is the drawback of the considerable size exhibited by the equipment, given that an induction furnace able to heat the size of ingot in question will have an external diameter of some 600 mm, to which the dimensions of the electrical panels must also be added. The size of the furnace is augmented further by being associated, necessarily, with an automatic or semi-automatic device for changing the ingot. The overall dimensions of the installation could be reduced somewhat by utilizing a single change device serving all the furnaces, though this would lead to notable structural complexities.
The prior art considers one particular multiple type of induction furnace, albeit designed for use with smaller ingots, especially in the transverse dimensions, which comprises a platform rotatable about a vertical axis and supporting a plurality of ingots spaced apart around the axis of rotation at equidistant intervals. Located above the platform is a support capable of movement in the vertical direction and carrying a plurality of open bottomed induction furnaces, the number of the induction furnaces being identical to the number of ingots carried by the platform. The support is designed to alternate between a lowered position in which the induction furnaces each encompass a relative ingot, the open bottom ends engaging in a close fit with the platform, and a raised position in which the platform is able to index through one angular step, corresponding to the distance between any two adjacent ingots. The furnaces are put into operation in such a way that they rotate around the axis of rotation and can be divided substantially into three zones of difference temperature, including one in which the temperature and the structure of the ingots is made uniform.
Not even this special multiple furnace can meet the requirements stated previously. However, such a furnace able to heat ingots of the dimensions indicated above would be unacceptable because of excessive dimensions and similarly excessive operating costs. Accordingly, the object of the present invention is to provide a method and equipment by means of which ingots can be heated to the semisolid or semiliquid state both swiftly and at reasonable cost.